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Welding and cutting processes used in shipbuilding
9 Welding and cutting processes used in shipbuilding Chapter Outline Gas welding 82 Electric arc welding
84
Slag-shielded processes 84 Manual welding electrodes 86 Automatic welding with cored wires 86 Submerged arc welding 86 Stud welding 88 Gas-shielded arc welding processes 89 Tungsten inert gas (TIG) welding 89 Metal inert gas (MIG) welding 89 Plasma welding 93
Other welding processes
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Electro-slag welding 94 Electro-gas welding 94 Laser welding 95 Thermit welding 96 Friction stir welding1 96
Cutting processes
98
Gas cutting 98 Plasma-arc cutting 98 Gouging 100 Laser cutting 100 Water jet cutting 101
Further reading 101 Some useful websites 101
Initially welding was used in ships as a means of repairing various metal parts. During the First World War various authorities connected with shipbuilding, including Lloyd’s Register, undertook research into welding and in some cases prototype welded structures were built. However, riveting remained the predominant method employed for joining ship plates and sections until the time of the Second World War. During and after this war the use and development of welding for shipbuilding purposes was widespread, and welding totally replaced riveting in the latter part of the twentieth century. Ship Construction. DOI: 10.1016/B978-0-08-097239-8.00009-X Copyright Ó 2012 Elsevier Ltd. All rights reserved.
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There are many advantages to be gained from employing welding in ships as opposed to having a riveted construction. These may be considered as advantages in both building and in operating the ship. For the shipbuilder the advantages are: 1. 2. 3. 4.
Welding lends itself to the adoption of prefabrication techniques. It is easier to obtain watertightness and oiltightness with welded joints. Joints are produced more quickly. Less skilled labor is required.
For the shipowner the advantages are: 1. Reduced hull steel weight, therefore more deadweight. 2. Less maintenance from slack rivets, etc. 3. The smoother hull with the elimination of overlapping plate joints leads to reduced skin friction resistance, which can reduce fuel costs.
Other than some blacksmith work involving solid-phase welding, the welding processes employed in shipbuilding are of the fusion welding type. Fusion welding is achieved by means of a heat source that is intense enough to melt the edges of the material to be joined as it is traversed along the joint. Gas welding, arc welding, laser welding, and resistance welding all provide heat sources of sufficient intensity to achieve fusion welds.
Gas welding A gas flame was probably the first form of heat source to be used for fusion welding, and a variety of fuel gases with oxygen have been used to produce a high-temperature flame. The most commonly used gas in use is acetylene, which gives an intense concentrated flame (average temperature 3000 C) when burnt in oxygen. An oxyacetylene flame has two distinct regions: an inner cone, in which the oxygen for combustion is supplied via the torch; and a surrounding envelope, in which some or all the oxygen for combustion is drawn from the surrounding air. By varying the ratio of oxygen to acetylene in the gas mixture supplied by the torch, it is possible to vary the efficiency of the combustion and alter the nature of the flame (Figure 9.1). If the oxygen supply is slightly greater than the supply of acetylene by volume, what is known as an ‘oxidizing’ flame is obtained. This type of flame may be used for welding materials of high thermal conductivity, e.g. copper, but not steels, as the steel may be decarburized and the weld pool depleted of silicon. With equal amounts of acetylene and oxygen a ‘neutral’ flame is obtained, and this would normally be used for welding steels and most other metals. Where the acetylene supply exceeds the oxygen by volume a ‘carburizing’ flame is obtained, the excess acetylene decomposing and producing submicroscopic particles of carbon. These readily go into solution in the molten steel, and can produce metallurgical problems in service. The outer envelope of the oxyacetylene flame by consuming the surrounding oxygen to some extent protects the molten weld metal pool from the surrounding air. If unprotected the oxygen may diffuse into the molten metal and produce porosity
Filler material
Torch
Weld metal
CARBURIZING FLAME
BACKWARD (OR LEFTWARD) WELDING
Welding and cutting processes used in shipbuilding
Weld direction
Weld direction NEUTRAL FLAME
Filler material
Torch
Weld metal FORWARD (OR RIGHTWARD) WELDING OXIDIZING FLAME
Figure 9.1 Gas welding.
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when the weld metal cools. With metals containing refractory oxides, such as stainless steels and aluminum, it is necessary to use an active flux to remove the oxides during the welding process. Both oxygen and acetylene are supplied in cylinders, the oxygen under pressure and the acetylene dissolved in acetone since it cannot be compressed. Each cylinder, which is distinctly colored (red—acetylene, black—oxygen), has a regulator for controlling the working gas pressures. The welding torch consists of a long thick copper nozzle, a gas mixer body, and valves for adjusting the oxygen and acetylene flow rates. Usually a welding rod is used to provide filler metal for the joint, but in some cases the parts to be joined may be fused together without any filler metal. Gas welding techniques are shown in Figure 9.1. Oxyacetylene welding tends to be slower than other fusion welding processes because the process temperature is low in comparison with the melting temperature of the metal, and because the heat must be transferred from the flame to the plate. The process is therefore only really applicable to thinner mild steel plate, thicknesses up to 7 mm being welded using this process with a speed of 3–4 meters per hour. In shipbuilding oxyacetylene welding has almost disappeared but can be employed in the fabrication of ventilation and air-conditioning trunking, cable trays, and light steel furniture; some plumbing and similar work may also make use of gas welding. These trades may also employ the gas flame for brazing purposes, where joints are obtained without reaching the fusion temperature of the material being joined.
Electric arc welding The basic principle of electric arc welding is that a wire or electrode is connected to a source of electrical supply with a return lead to the plates to be welded. If the electrode is brought into contact with the plates an electric current flows in the circuit. By removing the electrode a short distance from the plate, so that the electric current is able to jump the gap, a high-temperature electrical arc is created. This will melt the plate edges and the end of the electrode if this is of the consumable type. Electrical power sources vary, DC generators or rectifiers with variable or constant voltage characteristics being available, as well as AC transformers with variable voltage characteristics for single or multiple operation. The latter are most commonly used in shipbuilding. Illustrated in Figure 9.2 are the range of manual, semi-automatic, and automatic electric arc welding processes that might be employed in shipbuilding. Each of these electric arc welding processes is discussed below with its application.
Slag-shielded processes Metal arc welding started as bare wire welding, the wire being attached to normal power lines. This gave unsatisfactory welds, and subsequently it was discovered that by dipping the wire in lime a more stable arc was obtained. As a result of further developments many forms of flux are now available for coating the wire or for
Inert gas shielded
Nonconsumable electrode
Plasma arc
Slag shielded
Consumable electrode
Shielded Submerged Stud welding arc metal arc
Carbon Atomic TIG MIG MIG arc hydrogen argon argon CO2 shielded shielded shielded
Pulse transfer
Spray transfer
Welding and cutting processes used in shipbuilding
ELECTRIC ARC WELDING PROCESSES
Flux cored processes
Dip transfer
Figure 9.2 Electric arc welding processes.
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deposition on the joint prior to welding. Other developments include a hollow wire for continuous welding with the flux within the hollow core. The flux melts, then solidifies during the welding process, forming a solid slag that protects the weld from atmospheric oxygen and nitrogen.
Manual welding electrodes The core wire normally used for mild steel electrodes is rimming steel. This is ideal for wire-drawing purposes, and elements used to ‘kill’ steel such as silicon or aluminum tend to destabilize the arc, making ‘killed’ steels unsuitable. Coatings for the electrodes normally consist of a mixture of mineral silicates, oxides, fluorides, carbonates, hydrocarbons, and powdered metal alloys plus a liquid binder. After mixing, the coating is then extruded onto the core wire and the finished electrodes are dried in batches in ovens. Electrode coatings should provide gas shielding for the arc, easy striking and arc stability, a protective slag, good weld shape, and most important of all a gas shield consuming the surrounding oxygen and protecting the molten weld metal. Various electrode types are available, the type often being defined by the nature of the coating. The more important types are the rutile and basic (or low-hydrogen) electrodes. Rutile electrodes have coatings containing a high percentage of titania, and are generalpurpose electrodes that are easily controlled and give a good weld finish with sound properties. Basic or low-hydrogen electrodes, the coating of which has a high lime content, are manufactured with the moisture content of the coating reduced to a minimum to ensure low-hydrogen properties. The mechanical properties of weld metal deposited with this type of electrode are superior to those of other types, and basic electrodes are generally specified for welding the higher tensile strength steels. Where high restraint occurs, for example at the final erection seam weld between two athwartships rings of unit structure, low-hydrogen electrodes may also be employed. An experienced welder is required where this type of electrode is used since it is less easily controlled. Welding with manual electrodes may be accomplished in the downhand position, for example welding at the deck from above, also in the horizontal vertical, or vertical positions, for example across or up a bulkhead, and in the overhead position, for example welding at the deck from below (Figure 9.3). Welding in any of these positions requires selection of the correct electrode (positional suitability stipulated by the manufacturer), correct current, correct technique, and inevitably experience, particularly for the vertical and overhead positions.
Automatic welding with cored wires Flux-cored wires (FCAW) are often used in mechanized welding, allowing higher deposition rates and improved quality of weld. Basic or rutile flux-cored wires are commonly used for one-sided welding with a ceramic backing.
Submerged arc welding This is an arc welding process in which the arc is maintained within a blanket of granulated flux (see Figure 9.4). A consumable filler wire is employed and the arc is
Welding and cutting processes used in shipbuilding
Voltage control Generator
Current control A Electrode Arc
Workpiece
Electrode
DOWNHAND
Electrode
Weld Electrode Weld Electrode
Weld HORIZONTAL VERTICAL
Weld VERTICAL
OVERHEAD
Figure 9.3 Manual arc welding. 87
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SUBMERGED ARC WELDING Flux Wire feed Electrode wire
Run-on plate
Fused flux
Completed weld Flux ONE-SIDED WELDING TECHNIQUES
Flux retained by fireproof canvas
Ceramic backing
Air hose
Figure 9.4 Automatic arc welding.
maintained between this wire and the parent plate. Around the arc the granulated flux breaks down and provides some gases, and a highly protective thermally insulating molten container for the arc. This allows a high concentration of heat, making the process very efficient and suitable for heavy deposits at fast speeds. After welding the molten metal is protected by a layer of fused flux, which together with the unfused flux may be recovered before cooling. This is the most commonly used process for downhand mechanical welding in the shipbuilding industry, in particular for joining plates for ship shell, decks, and bulkheads. Metal powder additions that result in a 30–50% increase in metal deposition rate without incurring an increase in arc energy input may be used for the welding of joint thicknesses of 25 mm or more. Submerged arc multi-wire and twinarc systems are also used to give high productivity. With shipyards worldwide adopting one-side welding in their ship panel lines for improved productivity, the submerged arc process is commonly used with a fusible backing, using either flux or glass fiber materials to contain and control the weld penetration bead.
Stud welding Stud welding may be classed as a shielded arc process, the arc being drawn between the stud (electrode) and the plate to which the stud is to be attached. Each stud is inserted into a stud welding gun chuck, and a ceramic ferrule is slipped over it before the stud is placed against the plate surface. On depressing the gun trigger the stud is automatically retracted from the plate and the arc established, melting the end of the
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stud and the local plate surface. When the arcing period is complete, the current is automatically shut off and the stud driven into a molten pool of weld metal, so attaching stud to plate. Apart from the stud welding gun the equipment includes a control unit for timing the period of current flow. Granular flux is contained within the end of each stud to create a protective atmosphere during arcing. The ceramic ferrule that surrounds the weld area restricts the access of air to the weld zone; it also concentrates the heat of the arc and confines the molten metal to the weld area (see Figure 9.5). Stud welding is often used in shipbuilding, generally for the fastening of stud bolts to secure supports for pipe hangars, electric cable trays and other fittings, also insulation to bulkheads and wood sheathing to decks, etc. Apart from various forms of stud bolts, items like stud hooks and rings are also available.
Gas-shielded arc welding processes The application of bare wire welding with gas shielding was developed in the 1960s, and was quickly adopted for the welding of lighter steel structures in shipyards, as well as for welding aluminum alloys. Gas-shielded processes are principally of an automatic or semi-automatic nature.
Tungsten inert gas (TIG) welding In the TIG welding process the arc is drawn between a water-cooled nonconsumable tungsten electrode and the plate (Figure 9.6). An inert gas shield is provided to protect the weld metal from the atmosphere, and filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of a high-frequency discharge across the gap, since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally in Britain the inert gas shield used for welding aluminum and steel is argon. Only plate thicknesses of less than 6 mm would normally be welded by this process, and in particular aluminum sheet, a skilled operator being required for manual work. This may also be referred to as TAGS welding, i.e. tungsten arc gasshielded welding.
Metal inert gas (MIG) welding This is in effect an extension of TIG welding, the electrode in this process becoming a consumable metal wire. Basically the process is as illustrated in Figure 9.6, a wire feed motor supplying wire via guide rollers through a contact tube in the torch to the arc. An inert gas is supplied to the torch to shield the arc, and electrical connections are made to the contact tube and workpiece. Welding is almost always done with a DC source and electrode positive for regular metal transfer, and when welding aluminum to remove the oxide film by the action of the arc cathode. Although the process may be fully automatic, semi-automatic processes as illustrated with hand gun are now in greater use, and are particularly suitable in many cases for application to shipyard work.
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Stud Ceramic ferrule
Power source Plate Flux
Bent stud
Controller
Stud welding gun
Straight thread
Female thread
Control cable Welding cable
Stud
Plate WELDING CIRCUIT
Flux Flux
Figure 9.5 Stud welding.
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VARIOUS STUD TYPES
Wire feed Power source
Power source
Gas flow
Electrical contact tube
Gas flow Tungsten electrode
Cooling water
Continuously fed electrode wire
Arc
Arc
Welding and cutting processes used in shipbuilding
HF unit
METAL INERT GAS (MIG) PROCESS
TUNGSTEN INERT GAS (TIG) PROCESS
PRINCIPLE OF DIP TRANSFER 1
Arc established
2
3
Contact with plate
Resistance heating
5
4
Pinch effect
Arc re-established
Figure 9.6 Metal inert gas welding. 91
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Initially aluminum accounted for most of the MIG welding, with argon being used as the inert shielding gas. Much of the welding undertaken on aluminum deckhouses, and liquid methane gas tanks of specialized carriers, has made use of this process. Generally larger wire sizes and heavier currents have been employed in this work, metal transfer in the arc being by means of a spray transfer, i.e. metal droplets being projected at high speed across the arc. At low currents metal transfer in the arc is rather difficult and very little fusion of the plate results, which has made the welding of light aluminum plate rather difficult with the MIG/argon process. The introduction of the ‘pulsed arc’ process has to some extent overcome this problem and made positional welding easier. Here a low-level current is used with high-level pulses of current that detach the metal from the electrode and accelerate it across the arc to give good penetration. Early work on the welding of mild steel with the metal inert gas process made use of argon as a shielding gas, but as this gas is rather expensive, and satisfactory welding could only be accomplished in the downhand position, an alternative shielding gas was sought. Research in this direction was concentrated on the use of CO2 as the shielding gas, and the MIG/CO2 process is now widely used for welding mild steel. Using higher current values with thicker steel plate a fine spray transfer of the metal from the electrode across the arc is achieved, with a deep penetration. Wire diameters in excess of 1.6 mm are used, and currents above about 350 amps are required to obtain this form of transfer. Much of the higher current work is undertaken with automatic machines, but some semi-automatic torches are available to operate in this range in the hands of skilled welders. Welding is downhand only. On thinner plating where lower currents would be employed, a different mode of transfer of metal in the arc is achieved with the MIG/CO2 process. This form of welding is referred to as the dip transfer (or short-circuiting) process. The sequence of metal transfer is (see Figure 9.6): 1. 2. 3. 4. 5.
Establish the arc. Wire fed into arc until it makes contact with plate. Resistance heating of wire in contact with plate. Pinch effect, detaching heated portion of wire as droplet of molten metal. Re-establish the arc.
To prevent a rapid rise of current and ‘blast off’ of the end of the wire when it shortcircuits on the plate, variable inductance is introduced in the electrical circuit. Smaller wire diameters, 0.8 and 1.2 mm, are used where the dip transfer method is employed on lighter plate at low currents. The process is suitable for welding light mild steel plate in all positions. It may be used in shipbuilding as a semi-automatic process, particularly for welding deckhouses and other light steel assemblies. The pulsed MIG/argon process, developed for positional welding of light aluminum plate, may be used for positional welding of light steel plate but is likely to prove more expensive. Use of the MIG semi-automatic processes can considerably increase weld output and lower costs.
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This form of welding may also be collectively referred to as MAGS welding, i.e. metal arc gas-shielded welding.
Plasma welding This is very similar to TIG welding as the arc is formed between a pointed tungsten electrode and the plate. But, with the tungsten electrode positioned within the body of the torch, the plasma arc is separated from the shielding gas envelope (see Figure 9.7). Plasma is forced through a fine-bore copper nozzle that constricts the arc. By varying the bore diameter and plasma gas flow rate, three different operating modes can be achieved: 1. Microplasma—the arc is operated at very low welding currents (0.1–15 amps) and used for welding thin sheets (down to 0.1 mm thickness). 2. Medium current—the arc is operated at currents from 15 to 200 amps. Plasma welding is an alternative to conventional TIG welding, but with the advantage of achieving deeper penetration and having greater tolerance to surface contamination. Because of the bulkiness of the torch, it is more suited to mechanized welding than hand welding. 3. Keyhole plasma—the arc is operated at currents above 100 amps and by increasing the plasma flow a very powerful plasma beam is created. This can penetrate thicknesses up to 10 mm, but when using a single-pass technique is normally limited to a thickness of 6 mm. This operating mode is normally used for welding sheet metal (over 3 mm) in the downhand position.
Tungsten electrode Cooling water +ve
Plasma gas Power source Nozzle
–ve Shielding gas
Plate
Figure 9.7 Plasma welding.
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Other welding processes There are some welding processes that cannot strictly be classified as gas or arc welding processes and these are considered separately.
Electro-slag welding The electro-slag welding process is used for welding heavy casting structure components such as stern frames and was also used at an earlier stage to make vertical welds in heavier side shell when fabricated hull units where joined at the berth. With the development of the electro-gas welding process, electro-slag welding is no longer used for the latter purpose. To start the weld an arc is struck, but welding is achieved by resistance path heating through the flux, the initial arcing having been discouraged once welding is started. In Figure 9.8 the basic electro-slag process is illustrated; the current passes into the weld pool through the wire, and the copper water-cooled shoes retain the molten pool of weld metal. These may be mechanized so that they move up the plate as the weld is completed, flux being fed into the weld manually by the operator. A square edge preparation is used on the plates, and it is found that the final weld metal has a high plate dilution. ‘Run-on’ and ‘run-off’ plates are required for stopping and starting the weld, and it is desirable that the weld should be continuous. If a stoppage occurs it will be impossible to avoid a major slag inclusion in the weld, and it may then be necessary to cut out the original metal and start again. If very good weld properties are required with a fine grain structure (electro-slag welds tend to have a coarse grain structure) it is necessary to carry out a local normalizing treatment.
Electro-gas welding Of greater interest to the shipbuilder is a further development, electro-gas welding (see Figure 9.8). This is in fact an arc welding process that combines features of gasshielded welding with those of electro-slag welding. Water-cooled copper shoes similar to those for the electro-slag welding process are used, but a flux-cored wire rather than a bare wire is fed into the weld pool. Fusion is obtained by means of an arc established between the surface of the weld pool and the wire, and the CO2 or CO2 with argon mixture gas shield is supplied from separate nozzles or holes located centrally near the top of the copper shoes. The system is mechanized utilizing an automatic vertical-up welding machine fed by a power source and having a closed-loop cooling circuit and a level sensor that automatically adjusts the vertical travel speed. The process is more suitable for welding plates in the thickness range of 13–50 mm with square or vee edge preparations and is therefore used for shipbuilding purposes in the welding of vertical butts when erecting side shell panels or for the vertical shell butt joints when joining building blocks on the berth or dock. For this purpose the use of a single- or double-vee butt with the electro-gas process is preferable since this permits completion of the weld manually if any breakdown occurs. A square butt with appreciable gap would be almost impossible to bridge manually.
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Electrode wire Wire guide nozzle
Wire feed Arc
CO2
CO2
Water-cooled copper shoe ELECTRO-GAS WELDING Weld
Wire fed into pool
Weld direction
Weld pool Slag Molten metal
Copper shoe Completed weld
IN OUT
Cooling water
ELECTRO-SLAG WELDING
Figure 9.8 Electro-gas and electro-slag welding.
Laser welding Laser welding is being used in the shipbuilding industry and shows much promise as a welding process that offers low heat input and therefore minimum distortion of welded plates and stiffeners. For shipbuilding welding applications the laser source is either CO2 (see ‘Laser cutting’ section below) or Nd:YAG (neodimium–yttrium–aluminum–garnet) crystals. Because of the wide range of applied powers and power densities available from
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Nd:YAG lasers, different welding methods are possible. If the laser is in pulsed mode and the surface temperature is below boiling point, heat transfer is predominantly by conduction and a conduction limited weld is produced. If the applied power is higher (for a given speed), boiling begins in the weld pool and a deep penetration weld can be formed. After the pulse, the material flows back into the cavity and solidifies. Both these methods can be used to produce spot welds or ‘stake’ welds. Laser ‘stake welding’ has been used in shipbuilding, stiffening members being welded to plate panels from the plate side only. A seam weld is produced by a sequence of overlapping deep penetration ‘spot’ welds or by the formation of a continuous molten weld pool. Pulsed laser welding is normally used at thicknesses below about 3 mm. Higher power (4 to 10 kW) continuous-wave Nd:YAG lasers are capable of keyholetype welding materials from 0.8 to 15 mm thickness. Nd:YAG lasers can be used on a wide range of steels and aluminum alloys. Also, because of the possibility of using fiber-optic beam delivery, Nd:YAG lasers are often used in conjunction with articulated arm robots for welding fabricated units of complex shape. Since the beam can burn the skin or severely damage the eyes, Nd:YAG lasers require enclosures within the fabrication shop that are fully opaque to the Nd:YAG laser wavelength. Hybrid laser–arc welding is also used in the shipbuilding industry, this being a combination of laser and arc welding that produces deep penetration welds with good tolerance to poor joint fit-up. The Nd:YAG laser is combined with a metal arc welding gas (laser-MAG).
Thermit welding This is a very useful method of welding that may be used to weld together large steel sections, for example parts of a stern frame. It is in fact often used to repair castings or forgings of this nature. Thermit welding is basically a fusion process, the required heat being evolved from a mixture of powdered aluminum and iron oxide. The ends of the part to be welded are initially built into a sand or graphite mold, whilst the mixture is poured into a refractory lined crucible. Ignition of this mixture is obtained with the aid of a highly inflammable powder consisting mostly of barium peroxide. During the subsequent reaction within the crucible the oxygen leaves the iron oxide and combines with the aluminum producing aluminum oxide, or slag, and superheated thermit steel. This steel is run into the mold, where it preheats and eventually fuses and mixes with the ends of the parts to be joined. On cooling a continuous joint is formed and the mold is removed.
Friction stir welding1 Friction stir welding is a relatively new materials joining process that has been used in the shipbuilding industry and is likely to be more widely used. Friction stir welding is a solid-state process that offers advantages over fusion welding for certain applications. In producing butt joints it uses a nonconsumable rotating tool, the profiled pin of which is plunged into the butted joint of two plates 1
Friction stir welding was invented and patented by TWI Ltd., Cambridge, UK.
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and then moves along the joint. The plate material is softened in both plates and forced around the rotating profiled pin, resulting in a solid-state bond between the two plates (see Figure 9.9). To contain the softened material in the line of the joint, a backing bar is used and the tool shoulder under pressure retains material at the upper surface. Both plates and the backing bar require substantial clamping because of the forces involved. Plates of different thickness may be butt welded by inclining the rotating tool (Figure 9.9). The process is currently used for welding aluminum alloy plates and such plates to aluminum alloy extrusions or castings. Dissimilar aluminum alloys may be joined by the process. A suitable material for a rotating tool to permit friction stir welding of steel has yet to be developed. Typical applications of friction stir welding are the construction of aluminum alloy deck panels for high-speed craft from extruded sections and aluminum alloy honeycomb panels for passenger ship cabin bulkheads. Downward force Movement of tool Tool
Rotation of tool
Advancing side of weld
Shoulder
Probe
Retreating side of weld Shoulder
Height above backing bar
Figure 9.9 Friction stir welding.
Probe
Workpiece
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Cutting processes Steel plates and sections were mostly cut to shape in shipyards using a gas cutting technique, but the introduction of competitive plasma-arc cutting machines has led to their widespread use in shipyards today.
Gas cutting Gas cutting is achieved by what is basically a chemical/thermal reaction occurring with iron and iron alloys only. Iron or its alloys may be heated to a temperature at which the iron will rapidly oxidize in an atmosphere of high purity oxygen. The principle of the process as applied to the cutting of steel plates and sections in shipbuilding is as follows. Over a small area the metal is preheated to a given temperature, and a confined stream of oxygen is then blown onto this area. The iron is then oxidized in a narrow band, and the molten oxide and metal are removed by the kinetic energy of the oxygen stream. A narrow parallel sided gap is then left between the cut edges. Throughout the cutting operation the preheat flame is left on to heat the top of the cut since most of the heat produced by the reaction at the cutting front is not instantaneous, and tends to be liberated at the lower level of the cut only. Alloying elements in small amounts are dissolved in the slag and removed when cutting steel. However, if they are present in large quantities, alloying elements, especially chromium, will retard and even prevent cutting. The reason for this is that they either decrease the fluidity of the slag or produce a tenacious oxide film over the surface which prevents further oxidation of the iron. This may be overcome by introducing an iron rich powder into the cutting area, a process often referred to as ‘powder cutting’. When cutting stainless steels which have a high chromium content ‘powder cutting’ would be employed. Generally acetylene is used with oxygen to provide the preheat flame but other gases can be used: propane for example or hydrogen which is used for underwater work because of its compressibility. Apart from the torch, the equipment is similar to that for gas welding. The torch has valves for controlling the volume of acetylene and oxygen provided for the preheat flame, and it has a separate valve for controlling the oxygen jet (see Figure 9.10). The oxyacetylene cutting process has been highly automated for use in shipyards; these developments are considered in Chapter 13. Hand burning with an oxyacetylene flame is used extensively for small jobbing work, and during the fabrication and erection of units.
Plasma-arc cutting Plasma in this sense is a mass of ionized gas which will conduct electricity. An electrode is connected to the negative terminal of a DC supply and a gas shield is supplied for the arc from a nozzle which has a bore less than the natural diameter of the arc. As a result a constricted arc is obtained which has a temperature considerably higher than that of an open arc. The arc is established between the electrode and workpiece when the ionized conducting gas comes into contact with the work.
Tungsten electrode
VE
Torch nozzle
Plasma gas
Cut direction
Mild steel plate
+ VE
Water-cooled nozzle
Constricted arc
Side of kerf Plate
OXYACETYLENE CUTTING
1
2
PRINCIPLE OF PLASMA CUTTING TORCH
3
Oxygen supply
PLAN 30°
Coating
30°
Direction of cut
2 1
Welding and cutting processes used in shipbuilding
Oxygen cutting jet
Preheating flame mixture
Electrode
3
ELEVATION Bevel cut Plate
BURNING HEAD WITH THREE NOZZLES FOR PREPARING PLATE EDGE BEVELS
Surface preheated by arc
Combustion zone
ARC-AIR GOUGING
Figure 9.10 Metal cutting processes. 99
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This gas is ionized in the first place by a subsidiary electrical discharge between the electrode and the nozzle. Plates are cut by the high-temperature concentrated arc melting the material locally (Figure 9.10). The plasma-arc process may be used for cutting all electrically conductive materials. Cutting units are available with cutting currents of 20–1000 amps to cut plates with thicknesses of 0.6–150 mm. The plasma carrier gas may be compressed air, nitrogen, oxygen, or argon/hydrogen to cut mild or high alloy steels, and aluminum alloys, the more expensive argon/hydrogen mixture being required to cut the greater thickness sections. A water-injection plasma-arc cutting system is available for cutting materials up to 75 mm thick using nitrogen as the carrier gas. A higher cutting speed is possible and pollution minimized with the use of water and an exhaust system around the torch. Water cutting tables were often used with plasma-arc cutting, but more recent systems have dispensed with underwater cutting. Cutting in water absorbed the dust and particulate matter and reduced the plasma noise and ultraviolet radiation of earlier plasma cutters.
Gouging Both gas and arc welding processes may be modified to produce means of gouging out shallow depressions in plates to form edge preparations for welding purposes where precision is not important. Gouging is particularly useful in shipbuilding for cleaning out the backs of welds to expose clean metal prior to depositing a weld back run. The alternative to gouging for this task is mechanical chipping, which is slow and arduous. Usually, where gouging is applied for this purpose, what is known as ‘arc-air’ gouging is used. A tubular electrode is employed, the electrode metal conducting the current and maintaining an arc of sufficient intensity to heat the workpiece to incandescence. Whilst the arc is maintained, a stream of oxygen is discharged from the bore of the electrode that ignites the incandescent electrode metal and the combustible elements of the workpiece. At the same time the kinetic energy of the excess oxygen removes the products of combustion, and produces a cut. Held at an angle to the plate, the electrode will gouge out the unwanted material (Figures 9.8, 9.10). A gas cutting torch may be provided with special nozzles that allow gouging to be accomplished when the torch is held at an acute angle to the plate.
Laser cutting Profile cutting and planing at high speeds can be achieved with a concentrated laser beam and has increasingly been employed in a mechanized or robotic form in the shipbuilding industry in recent years. In a laser beam the light is of one wavelength, travels in the same direction, and is coherent, i.e. all the waves are in phase. Such a beam can be focused to give high energy densities. For welding and cutting the beam is generated in a CO2 laser. This consists of a tube filled with a mixture of CO2, nitrogen, and helium that is made to fluoresce by a high-voltage discharge. The tube emits infrared radiation with a wavelength of about 1.6 mm and is capable of delivering outputs up to 20 kW.
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Laser cutting relies on keyholing to penetrate the thickness, and the molten metal is blown out of the hole by a gas jet. A nozzle is fitted concentric with the output from a CO2 laser so that a gas jet can be directed at the work coaxial with the laser beam. The jet can be an inert gas, nitrogen, or in the case of steel, oxygen. With oxygen there is an exothermic reaction with the steel, giving additional heat as in oxy-fuel cutting. The thermal keyholing gives a narrow straight-sided cut compared with the normal cut obtained by other processes relying on a chemical reaction.
Water jet cutting The cutting tool employed in this process is a concentrated water jet, with or without abrasive, which is released from a nozzle at 2½ times the speed of sound and at a pressure level of several thousand bar. Water jet cutting can be used on a range of materials such as timber, plastics, rubber, etc., as well as steels and aluminum alloys. Mild steel from 0.25 to 150 mm in thickness and aluminum alloys from 0.5 to 250 mm in thickness can be cut. Being a cold cutting process the heat-affected zone, mechanical stresses, and distortion are left at the cut surface. Water jet cutting is slower than most thermal cutting processes and is not a portable machine tool.
Further reading ESAB advances in welding and cutting technology, The Naval Architect, July/August 2003. Exploiting friction-stir welding of aluminium, The Naval Architect, July/August 2004. Modern materials and processes for shipbuilding, The Naval Architect, July/August 2005. Smith BD: Welding Practice, 1996, Butterworth Heinemann.
Some useful websites http://www.rina.org.uk/tna.html Site for The Naval Architect magazine, with regular updates on ship and shipbuilding technology, including welding. www.esab.com Recommended for informative papers on welding processes and practices. www.controlwaterjet.co.uk Use, benefits, and capabilities of water jet cutting.
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Slag-shielded processes 84 Manual welding electrodes 86 Automatic welding with cored wires 86 Submerged arc welding 86 Stud welding 88 Gas-shielded arc welding processes 89 Tungsten inert gas (TIG) welding 89 Metal inert gas (MIG) welding 89 Plasma welding 93
Other welding processes
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Electro-slag welding 94 Electro-gas welding 94 Laser welding 95 Thermit welding 96 Friction stir welding1 96
Cutting processes
98
Gas cutting 98 Plasma-arc cutting 98 Gouging 100 Laser cutting 100 Water jet cutting 101
Further reading 101 Some useful websites 101
Initially welding was used in ships as a means of repairing various metal parts. During the First World War various authorities connected with shipbuilding, including Lloyd’s Register, undertook research into welding and in some cases prototype welded structures were built. However, riveting remained the predominant method employed for joining ship plates and sections until the time of the Second World War. During and after this war the use and development of welding for shipbuilding purposes was widespread, and welding totally replaced riveting in the latter part of the twentieth century. Ship Construction. DOI: 10.1016/B978-0-08-097239-8.00009-X Copyright Ó 2012 Elsevier Ltd. All rights reserved.
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There are many advantages to be gained from employing welding in ships as opposed to having a riveted construction. These may be considered as advantages in both building and in operating the ship. For the shipbuilder the advantages are: 1. 2. 3. 4.
Welding lends itself to the adoption of prefabrication techniques. It is easier to obtain watertightness and oiltightness with welded joints. Joints are produced more quickly. Less skilled labor is required.
For the shipowner the advantages are: 1. Reduced hull steel weight, therefore more deadweight. 2. Less maintenance from slack rivets, etc. 3. The smoother hull with the elimination of overlapping plate joints leads to reduced skin friction resistance, which can reduce fuel costs.
Other than some blacksmith work involving solid-phase welding, the welding processes employed in shipbuilding are of the fusion welding type. Fusion welding is achieved by means of a heat source that is intense enough to melt the edges of the material to be joined as it is traversed along the joint. Gas welding, arc welding, laser welding, and resistance welding all provide heat sources of sufficient intensity to achieve fusion welds.
Gas welding A gas flame was probably the first form of heat source to be used for fusion welding, and a variety of fuel gases with oxygen have been used to produce a high-temperature flame. The most commonly used gas in use is acetylene, which gives an intense concentrated flame (average temperature 3000 C) when burnt in oxygen. An oxyacetylene flame has two distinct regions: an inner cone, in which the oxygen for combustion is supplied via the torch; and a surrounding envelope, in which some or all the oxygen for combustion is drawn from the surrounding air. By varying the ratio of oxygen to acetylene in the gas mixture supplied by the torch, it is possible to vary the efficiency of the combustion and alter the nature of the flame (Figure 9.1). If the oxygen supply is slightly greater than the supply of acetylene by volume, what is known as an ‘oxidizing’ flame is obtained. This type of flame may be used for welding materials of high thermal conductivity, e.g. copper, but not steels, as the steel may be decarburized and the weld pool depleted of silicon. With equal amounts of acetylene and oxygen a ‘neutral’ flame is obtained, and this would normally be used for welding steels and most other metals. Where the acetylene supply exceeds the oxygen by volume a ‘carburizing’ flame is obtained, the excess acetylene decomposing and producing submicroscopic particles of carbon. These readily go into solution in the molten steel, and can produce metallurgical problems in service. The outer envelope of the oxyacetylene flame by consuming the surrounding oxygen to some extent protects the molten weld metal pool from the surrounding air. If unprotected the oxygen may diffuse into the molten metal and produce porosity
Filler material
Torch
Weld metal
CARBURIZING FLAME
BACKWARD (OR LEFTWARD) WELDING
Welding and cutting processes used in shipbuilding
Weld direction
Weld direction NEUTRAL FLAME
Filler material
Torch
Weld metal FORWARD (OR RIGHTWARD) WELDING OXIDIZING FLAME
Figure 9.1 Gas welding.
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when the weld metal cools. With metals containing refractory oxides, such as stainless steels and aluminum, it is necessary to use an active flux to remove the oxides during the welding process. Both oxygen and acetylene are supplied in cylinders, the oxygen under pressure and the acetylene dissolved in acetone since it cannot be compressed. Each cylinder, which is distinctly colored (red—acetylene, black—oxygen), has a regulator for controlling the working gas pressures. The welding torch consists of a long thick copper nozzle, a gas mixer body, and valves for adjusting the oxygen and acetylene flow rates. Usually a welding rod is used to provide filler metal for the joint, but in some cases the parts to be joined may be fused together without any filler metal. Gas welding techniques are shown in Figure 9.1. Oxyacetylene welding tends to be slower than other fusion welding processes because the process temperature is low in comparison with the melting temperature of the metal, and because the heat must be transferred from the flame to the plate. The process is therefore only really applicable to thinner mild steel plate, thicknesses up to 7 mm being welded using this process with a speed of 3–4 meters per hour. In shipbuilding oxyacetylene welding has almost disappeared but can be employed in the fabrication of ventilation and air-conditioning trunking, cable trays, and light steel furniture; some plumbing and similar work may also make use of gas welding. These trades may also employ the gas flame for brazing purposes, where joints are obtained without reaching the fusion temperature of the material being joined.
Electric arc welding The basic principle of electric arc welding is that a wire or electrode is connected to a source of electrical supply with a return lead to the plates to be welded. If the electrode is brought into contact with the plates an electric current flows in the circuit. By removing the electrode a short distance from the plate, so that the electric current is able to jump the gap, a high-temperature electrical arc is created. This will melt the plate edges and the end of the electrode if this is of the consumable type. Electrical power sources vary, DC generators or rectifiers with variable or constant voltage characteristics being available, as well as AC transformers with variable voltage characteristics for single or multiple operation. The latter are most commonly used in shipbuilding. Illustrated in Figure 9.2 are the range of manual, semi-automatic, and automatic electric arc welding processes that might be employed in shipbuilding. Each of these electric arc welding processes is discussed below with its application.
Slag-shielded processes Metal arc welding started as bare wire welding, the wire being attached to normal power lines. This gave unsatisfactory welds, and subsequently it was discovered that by dipping the wire in lime a more stable arc was obtained. As a result of further developments many forms of flux are now available for coating the wire or for
Inert gas shielded
Nonconsumable electrode
Plasma arc
Slag shielded
Consumable electrode
Shielded Submerged Stud welding arc metal arc
Carbon Atomic TIG MIG MIG arc hydrogen argon argon CO2 shielded shielded shielded
Pulse transfer
Spray transfer
Welding and cutting processes used in shipbuilding
ELECTRIC ARC WELDING PROCESSES
Flux cored processes
Dip transfer
Figure 9.2 Electric arc welding processes.
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deposition on the joint prior to welding. Other developments include a hollow wire for continuous welding with the flux within the hollow core. The flux melts, then solidifies during the welding process, forming a solid slag that protects the weld from atmospheric oxygen and nitrogen.
Manual welding electrodes The core wire normally used for mild steel electrodes is rimming steel. This is ideal for wire-drawing purposes, and elements used to ‘kill’ steel such as silicon or aluminum tend to destabilize the arc, making ‘killed’ steels unsuitable. Coatings for the electrodes normally consist of a mixture of mineral silicates, oxides, fluorides, carbonates, hydrocarbons, and powdered metal alloys plus a liquid binder. After mixing, the coating is then extruded onto the core wire and the finished electrodes are dried in batches in ovens. Electrode coatings should provide gas shielding for the arc, easy striking and arc stability, a protective slag, good weld shape, and most important of all a gas shield consuming the surrounding oxygen and protecting the molten weld metal. Various electrode types are available, the type often being defined by the nature of the coating. The more important types are the rutile and basic (or low-hydrogen) electrodes. Rutile electrodes have coatings containing a high percentage of titania, and are generalpurpose electrodes that are easily controlled and give a good weld finish with sound properties. Basic or low-hydrogen electrodes, the coating of which has a high lime content, are manufactured with the moisture content of the coating reduced to a minimum to ensure low-hydrogen properties. The mechanical properties of weld metal deposited with this type of electrode are superior to those of other types, and basic electrodes are generally specified for welding the higher tensile strength steels. Where high restraint occurs, for example at the final erection seam weld between two athwartships rings of unit structure, low-hydrogen electrodes may also be employed. An experienced welder is required where this type of electrode is used since it is less easily controlled. Welding with manual electrodes may be accomplished in the downhand position, for example welding at the deck from above, also in the horizontal vertical, or vertical positions, for example across or up a bulkhead, and in the overhead position, for example welding at the deck from below (Figure 9.3). Welding in any of these positions requires selection of the correct electrode (positional suitability stipulated by the manufacturer), correct current, correct technique, and inevitably experience, particularly for the vertical and overhead positions.
Automatic welding with cored wires Flux-cored wires (FCAW) are often used in mechanized welding, allowing higher deposition rates and improved quality of weld. Basic or rutile flux-cored wires are commonly used for one-sided welding with a ceramic backing.
Submerged arc welding This is an arc welding process in which the arc is maintained within a blanket of granulated flux (see Figure 9.4). A consumable filler wire is employed and the arc is
Welding and cutting processes used in shipbuilding
Voltage control Generator
Current control A Electrode Arc
Workpiece
Electrode
DOWNHAND
Electrode
Weld Electrode Weld Electrode
Weld HORIZONTAL VERTICAL
Weld VERTICAL
OVERHEAD
Figure 9.3 Manual arc welding. 87
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SUBMERGED ARC WELDING Flux Wire feed Electrode wire
Run-on plate
Fused flux
Completed weld Flux ONE-SIDED WELDING TECHNIQUES
Flux retained by fireproof canvas
Ceramic backing
Air hose
Figure 9.4 Automatic arc welding.
maintained between this wire and the parent plate. Around the arc the granulated flux breaks down and provides some gases, and a highly protective thermally insulating molten container for the arc. This allows a high concentration of heat, making the process very efficient and suitable for heavy deposits at fast speeds. After welding the molten metal is protected by a layer of fused flux, which together with the unfused flux may be recovered before cooling. This is the most commonly used process for downhand mechanical welding in the shipbuilding industry, in particular for joining plates for ship shell, decks, and bulkheads. Metal powder additions that result in a 30–50% increase in metal deposition rate without incurring an increase in arc energy input may be used for the welding of joint thicknesses of 25 mm or more. Submerged arc multi-wire and twinarc systems are also used to give high productivity. With shipyards worldwide adopting one-side welding in their ship panel lines for improved productivity, the submerged arc process is commonly used with a fusible backing, using either flux or glass fiber materials to contain and control the weld penetration bead.
Stud welding Stud welding may be classed as a shielded arc process, the arc being drawn between the stud (electrode) and the plate to which the stud is to be attached. Each stud is inserted into a stud welding gun chuck, and a ceramic ferrule is slipped over it before the stud is placed against the plate surface. On depressing the gun trigger the stud is automatically retracted from the plate and the arc established, melting the end of the
Welding and cutting processes used in shipbuilding
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stud and the local plate surface. When the arcing period is complete, the current is automatically shut off and the stud driven into a molten pool of weld metal, so attaching stud to plate. Apart from the stud welding gun the equipment includes a control unit for timing the period of current flow. Granular flux is contained within the end of each stud to create a protective atmosphere during arcing. The ceramic ferrule that surrounds the weld area restricts the access of air to the weld zone; it also concentrates the heat of the arc and confines the molten metal to the weld area (see Figure 9.5). Stud welding is often used in shipbuilding, generally for the fastening of stud bolts to secure supports for pipe hangars, electric cable trays and other fittings, also insulation to bulkheads and wood sheathing to decks, etc. Apart from various forms of stud bolts, items like stud hooks and rings are also available.
Gas-shielded arc welding processes The application of bare wire welding with gas shielding was developed in the 1960s, and was quickly adopted for the welding of lighter steel structures in shipyards, as well as for welding aluminum alloys. Gas-shielded processes are principally of an automatic or semi-automatic nature.
Tungsten inert gas (TIG) welding In the TIG welding process the arc is drawn between a water-cooled nonconsumable tungsten electrode and the plate (Figure 9.6). An inert gas shield is provided to protect the weld metal from the atmosphere, and filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of a high-frequency discharge across the gap, since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally in Britain the inert gas shield used for welding aluminum and steel is argon. Only plate thicknesses of less than 6 mm would normally be welded by this process, and in particular aluminum sheet, a skilled operator being required for manual work. This may also be referred to as TAGS welding, i.e. tungsten arc gasshielded welding.
Metal inert gas (MIG) welding This is in effect an extension of TIG welding, the electrode in this process becoming a consumable metal wire. Basically the process is as illustrated in Figure 9.6, a wire feed motor supplying wire via guide rollers through a contact tube in the torch to the arc. An inert gas is supplied to the torch to shield the arc, and electrical connections are made to the contact tube and workpiece. Welding is almost always done with a DC source and electrode positive for regular metal transfer, and when welding aluminum to remove the oxide film by the action of the arc cathode. Although the process may be fully automatic, semi-automatic processes as illustrated with hand gun are now in greater use, and are particularly suitable in many cases for application to shipyard work.
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Stud Ceramic ferrule
Power source Plate Flux
Bent stud
Controller
Stud welding gun
Straight thread
Female thread
Control cable Welding cable
Stud
Plate WELDING CIRCUIT
Flux Flux
Figure 9.5 Stud welding.
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VARIOUS STUD TYPES
Wire feed Power source
Power source
Gas flow
Electrical contact tube
Gas flow Tungsten electrode
Cooling water
Continuously fed electrode wire
Arc
Arc
Welding and cutting processes used in shipbuilding
HF unit
METAL INERT GAS (MIG) PROCESS
TUNGSTEN INERT GAS (TIG) PROCESS
PRINCIPLE OF DIP TRANSFER 1
Arc established
2
3
Contact with plate
Resistance heating
5
4
Pinch effect
Arc re-established
Figure 9.6 Metal inert gas welding. 91
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Initially aluminum accounted for most of the MIG welding, with argon being used as the inert shielding gas. Much of the welding undertaken on aluminum deckhouses, and liquid methane gas tanks of specialized carriers, has made use of this process. Generally larger wire sizes and heavier currents have been employed in this work, metal transfer in the arc being by means of a spray transfer, i.e. metal droplets being projected at high speed across the arc. At low currents metal transfer in the arc is rather difficult and very little fusion of the plate results, which has made the welding of light aluminum plate rather difficult with the MIG/argon process. The introduction of the ‘pulsed arc’ process has to some extent overcome this problem and made positional welding easier. Here a low-level current is used with high-level pulses of current that detach the metal from the electrode and accelerate it across the arc to give good penetration. Early work on the welding of mild steel with the metal inert gas process made use of argon as a shielding gas, but as this gas is rather expensive, and satisfactory welding could only be accomplished in the downhand position, an alternative shielding gas was sought. Research in this direction was concentrated on the use of CO2 as the shielding gas, and the MIG/CO2 process is now widely used for welding mild steel. Using higher current values with thicker steel plate a fine spray transfer of the metal from the electrode across the arc is achieved, with a deep penetration. Wire diameters in excess of 1.6 mm are used, and currents above about 350 amps are required to obtain this form of transfer. Much of the higher current work is undertaken with automatic machines, but some semi-automatic torches are available to operate in this range in the hands of skilled welders. Welding is downhand only. On thinner plating where lower currents would be employed, a different mode of transfer of metal in the arc is achieved with the MIG/CO2 process. This form of welding is referred to as the dip transfer (or short-circuiting) process. The sequence of metal transfer is (see Figure 9.6): 1. 2. 3. 4. 5.
Establish the arc. Wire fed into arc until it makes contact with plate. Resistance heating of wire in contact with plate. Pinch effect, detaching heated portion of wire as droplet of molten metal. Re-establish the arc.
To prevent a rapid rise of current and ‘blast off’ of the end of the wire when it shortcircuits on the plate, variable inductance is introduced in the electrical circuit. Smaller wire diameters, 0.8 and 1.2 mm, are used where the dip transfer method is employed on lighter plate at low currents. The process is suitable for welding light mild steel plate in all positions. It may be used in shipbuilding as a semi-automatic process, particularly for welding deckhouses and other light steel assemblies. The pulsed MIG/argon process, developed for positional welding of light aluminum plate, may be used for positional welding of light steel plate but is likely to prove more expensive. Use of the MIG semi-automatic processes can considerably increase weld output and lower costs.
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This form of welding may also be collectively referred to as MAGS welding, i.e. metal arc gas-shielded welding.
Plasma welding This is very similar to TIG welding as the arc is formed between a pointed tungsten electrode and the plate. But, with the tungsten electrode positioned within the body of the torch, the plasma arc is separated from the shielding gas envelope (see Figure 9.7). Plasma is forced through a fine-bore copper nozzle that constricts the arc. By varying the bore diameter and plasma gas flow rate, three different operating modes can be achieved: 1. Microplasma—the arc is operated at very low welding currents (0.1–15 amps) and used for welding thin sheets (down to 0.1 mm thickness). 2. Medium current—the arc is operated at currents from 15 to 200 amps. Plasma welding is an alternative to conventional TIG welding, but with the advantage of achieving deeper penetration and having greater tolerance to surface contamination. Because of the bulkiness of the torch, it is more suited to mechanized welding than hand welding. 3. Keyhole plasma—the arc is operated at currents above 100 amps and by increasing the plasma flow a very powerful plasma beam is created. This can penetrate thicknesses up to 10 mm, but when using a single-pass technique is normally limited to a thickness of 6 mm. This operating mode is normally used for welding sheet metal (over 3 mm) in the downhand position.
Tungsten electrode Cooling water +ve
Plasma gas Power source Nozzle
–ve Shielding gas
Plate
Figure 9.7 Plasma welding.
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Other welding processes There are some welding processes that cannot strictly be classified as gas or arc welding processes and these are considered separately.
Electro-slag welding The electro-slag welding process is used for welding heavy casting structure components such as stern frames and was also used at an earlier stage to make vertical welds in heavier side shell when fabricated hull units where joined at the berth. With the development of the electro-gas welding process, electro-slag welding is no longer used for the latter purpose. To start the weld an arc is struck, but welding is achieved by resistance path heating through the flux, the initial arcing having been discouraged once welding is started. In Figure 9.8 the basic electro-slag process is illustrated; the current passes into the weld pool through the wire, and the copper water-cooled shoes retain the molten pool of weld metal. These may be mechanized so that they move up the plate as the weld is completed, flux being fed into the weld manually by the operator. A square edge preparation is used on the plates, and it is found that the final weld metal has a high plate dilution. ‘Run-on’ and ‘run-off’ plates are required for stopping and starting the weld, and it is desirable that the weld should be continuous. If a stoppage occurs it will be impossible to avoid a major slag inclusion in the weld, and it may then be necessary to cut out the original metal and start again. If very good weld properties are required with a fine grain structure (electro-slag welds tend to have a coarse grain structure) it is necessary to carry out a local normalizing treatment.
Electro-gas welding Of greater interest to the shipbuilder is a further development, electro-gas welding (see Figure 9.8). This is in fact an arc welding process that combines features of gasshielded welding with those of electro-slag welding. Water-cooled copper shoes similar to those for the electro-slag welding process are used, but a flux-cored wire rather than a bare wire is fed into the weld pool. Fusion is obtained by means of an arc established between the surface of the weld pool and the wire, and the CO2 or CO2 with argon mixture gas shield is supplied from separate nozzles or holes located centrally near the top of the copper shoes. The system is mechanized utilizing an automatic vertical-up welding machine fed by a power source and having a closed-loop cooling circuit and a level sensor that automatically adjusts the vertical travel speed. The process is more suitable for welding plates in the thickness range of 13–50 mm with square or vee edge preparations and is therefore used for shipbuilding purposes in the welding of vertical butts when erecting side shell panels or for the vertical shell butt joints when joining building blocks on the berth or dock. For this purpose the use of a single- or double-vee butt with the electro-gas process is preferable since this permits completion of the weld manually if any breakdown occurs. A square butt with appreciable gap would be almost impossible to bridge manually.
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Electrode wire Wire guide nozzle
Wire feed Arc
CO2
CO2
Water-cooled copper shoe ELECTRO-GAS WELDING Weld
Wire fed into pool
Weld direction
Weld pool Slag Molten metal
Copper shoe Completed weld
IN OUT
Cooling water
ELECTRO-SLAG WELDING
Figure 9.8 Electro-gas and electro-slag welding.
Laser welding Laser welding is being used in the shipbuilding industry and shows much promise as a welding process that offers low heat input and therefore minimum distortion of welded plates and stiffeners. For shipbuilding welding applications the laser source is either CO2 (see ‘Laser cutting’ section below) or Nd:YAG (neodimium–yttrium–aluminum–garnet) crystals. Because of the wide range of applied powers and power densities available from
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Nd:YAG lasers, different welding methods are possible. If the laser is in pulsed mode and the surface temperature is below boiling point, heat transfer is predominantly by conduction and a conduction limited weld is produced. If the applied power is higher (for a given speed), boiling begins in the weld pool and a deep penetration weld can be formed. After the pulse, the material flows back into the cavity and solidifies. Both these methods can be used to produce spot welds or ‘stake’ welds. Laser ‘stake welding’ has been used in shipbuilding, stiffening members being welded to plate panels from the plate side only. A seam weld is produced by a sequence of overlapping deep penetration ‘spot’ welds or by the formation of a continuous molten weld pool. Pulsed laser welding is normally used at thicknesses below about 3 mm. Higher power (4 to 10 kW) continuous-wave Nd:YAG lasers are capable of keyholetype welding materials from 0.8 to 15 mm thickness. Nd:YAG lasers can be used on a wide range of steels and aluminum alloys. Also, because of the possibility of using fiber-optic beam delivery, Nd:YAG lasers are often used in conjunction with articulated arm robots for welding fabricated units of complex shape. Since the beam can burn the skin or severely damage the eyes, Nd:YAG lasers require enclosures within the fabrication shop that are fully opaque to the Nd:YAG laser wavelength. Hybrid laser–arc welding is also used in the shipbuilding industry, this being a combination of laser and arc welding that produces deep penetration welds with good tolerance to poor joint fit-up. The Nd:YAG laser is combined with a metal arc welding gas (laser-MAG).
Thermit welding This is a very useful method of welding that may be used to weld together large steel sections, for example parts of a stern frame. It is in fact often used to repair castings or forgings of this nature. Thermit welding is basically a fusion process, the required heat being evolved from a mixture of powdered aluminum and iron oxide. The ends of the part to be welded are initially built into a sand or graphite mold, whilst the mixture is poured into a refractory lined crucible. Ignition of this mixture is obtained with the aid of a highly inflammable powder consisting mostly of barium peroxide. During the subsequent reaction within the crucible the oxygen leaves the iron oxide and combines with the aluminum producing aluminum oxide, or slag, and superheated thermit steel. This steel is run into the mold, where it preheats and eventually fuses and mixes with the ends of the parts to be joined. On cooling a continuous joint is formed and the mold is removed.
Friction stir welding1 Friction stir welding is a relatively new materials joining process that has been used in the shipbuilding industry and is likely to be more widely used. Friction stir welding is a solid-state process that offers advantages over fusion welding for certain applications. In producing butt joints it uses a nonconsumable rotating tool, the profiled pin of which is plunged into the butted joint of two plates 1
Friction stir welding was invented and patented by TWI Ltd., Cambridge, UK.
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and then moves along the joint. The plate material is softened in both plates and forced around the rotating profiled pin, resulting in a solid-state bond between the two plates (see Figure 9.9). To contain the softened material in the line of the joint, a backing bar is used and the tool shoulder under pressure retains material at the upper surface. Both plates and the backing bar require substantial clamping because of the forces involved. Plates of different thickness may be butt welded by inclining the rotating tool (Figure 9.9). The process is currently used for welding aluminum alloy plates and such plates to aluminum alloy extrusions or castings. Dissimilar aluminum alloys may be joined by the process. A suitable material for a rotating tool to permit friction stir welding of steel has yet to be developed. Typical applications of friction stir welding are the construction of aluminum alloy deck panels for high-speed craft from extruded sections and aluminum alloy honeycomb panels for passenger ship cabin bulkheads. Downward force Movement of tool Tool
Rotation of tool
Advancing side of weld
Shoulder
Probe
Retreating side of weld Shoulder
Height above backing bar
Figure 9.9 Friction stir welding.
Probe
Workpiece
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Cutting processes Steel plates and sections were mostly cut to shape in shipyards using a gas cutting technique, but the introduction of competitive plasma-arc cutting machines has led to their widespread use in shipyards today.
Gas cutting Gas cutting is achieved by what is basically a chemical/thermal reaction occurring with iron and iron alloys only. Iron or its alloys may be heated to a temperature at which the iron will rapidly oxidize in an atmosphere of high purity oxygen. The principle of the process as applied to the cutting of steel plates and sections in shipbuilding is as follows. Over a small area the metal is preheated to a given temperature, and a confined stream of oxygen is then blown onto this area. The iron is then oxidized in a narrow band, and the molten oxide and metal are removed by the kinetic energy of the oxygen stream. A narrow parallel sided gap is then left between the cut edges. Throughout the cutting operation the preheat flame is left on to heat the top of the cut since most of the heat produced by the reaction at the cutting front is not instantaneous, and tends to be liberated at the lower level of the cut only. Alloying elements in small amounts are dissolved in the slag and removed when cutting steel. However, if they are present in large quantities, alloying elements, especially chromium, will retard and even prevent cutting. The reason for this is that they either decrease the fluidity of the slag or produce a tenacious oxide film over the surface which prevents further oxidation of the iron. This may be overcome by introducing an iron rich powder into the cutting area, a process often referred to as ‘powder cutting’. When cutting stainless steels which have a high chromium content ‘powder cutting’ would be employed. Generally acetylene is used with oxygen to provide the preheat flame but other gases can be used: propane for example or hydrogen which is used for underwater work because of its compressibility. Apart from the torch, the equipment is similar to that for gas welding. The torch has valves for controlling the volume of acetylene and oxygen provided for the preheat flame, and it has a separate valve for controlling the oxygen jet (see Figure 9.10). The oxyacetylene cutting process has been highly automated for use in shipyards; these developments are considered in Chapter 13. Hand burning with an oxyacetylene flame is used extensively for small jobbing work, and during the fabrication and erection of units.
Plasma-arc cutting Plasma in this sense is a mass of ionized gas which will conduct electricity. An electrode is connected to the negative terminal of a DC supply and a gas shield is supplied for the arc from a nozzle which has a bore less than the natural diameter of the arc. As a result a constricted arc is obtained which has a temperature considerably higher than that of an open arc. The arc is established between the electrode and workpiece when the ionized conducting gas comes into contact with the work.
Tungsten electrode
VE
Torch nozzle
Plasma gas
Cut direction
Mild steel plate
+ VE
Water-cooled nozzle
Constricted arc
Side of kerf Plate
OXYACETYLENE CUTTING
1
2
PRINCIPLE OF PLASMA CUTTING TORCH
3
Oxygen supply
PLAN 30°
Coating
30°
Direction of cut
2 1
Welding and cutting processes used in shipbuilding
Oxygen cutting jet
Preheating flame mixture
Electrode
3
ELEVATION Bevel cut Plate
BURNING HEAD WITH THREE NOZZLES FOR PREPARING PLATE EDGE BEVELS
Surface preheated by arc
Combustion zone
ARC-AIR GOUGING
Figure 9.10 Metal cutting processes. 99
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Ship Construction
This gas is ionized in the first place by a subsidiary electrical discharge between the electrode and the nozzle. Plates are cut by the high-temperature concentrated arc melting the material locally (Figure 9.10). The plasma-arc process may be used for cutting all electrically conductive materials. Cutting units are available with cutting currents of 20–1000 amps to cut plates with thicknesses of 0.6–150 mm. The plasma carrier gas may be compressed air, nitrogen, oxygen, or argon/hydrogen to cut mild or high alloy steels, and aluminum alloys, the more expensive argon/hydrogen mixture being required to cut the greater thickness sections. A water-injection plasma-arc cutting system is available for cutting materials up to 75 mm thick using nitrogen as the carrier gas. A higher cutting speed is possible and pollution minimized with the use of water and an exhaust system around the torch. Water cutting tables were often used with plasma-arc cutting, but more recent systems have dispensed with underwater cutting. Cutting in water absorbed the dust and particulate matter and reduced the plasma noise and ultraviolet radiation of earlier plasma cutters.
Gouging Both gas and arc welding processes may be modified to produce means of gouging out shallow depressions in plates to form edge preparations for welding purposes where precision is not important. Gouging is particularly useful in shipbuilding for cleaning out the backs of welds to expose clean metal prior to depositing a weld back run. The alternative to gouging for this task is mechanical chipping, which is slow and arduous. Usually, where gouging is applied for this purpose, what is known as ‘arc-air’ gouging is used. A tubular electrode is employed, the electrode metal conducting the current and maintaining an arc of sufficient intensity to heat the workpiece to incandescence. Whilst the arc is maintained, a stream of oxygen is discharged from the bore of the electrode that ignites the incandescent electrode metal and the combustible elements of the workpiece. At the same time the kinetic energy of the excess oxygen removes the products of combustion, and produces a cut. Held at an angle to the plate, the electrode will gouge out the unwanted material (Figures 9.8, 9.10). A gas cutting torch may be provided with special nozzles that allow gouging to be accomplished when the torch is held at an acute angle to the plate.
Laser cutting Profile cutting and planing at high speeds can be achieved with a concentrated laser beam and has increasingly been employed in a mechanized or robotic form in the shipbuilding industry in recent years. In a laser beam the light is of one wavelength, travels in the same direction, and is coherent, i.e. all the waves are in phase. Such a beam can be focused to give high energy densities. For welding and cutting the beam is generated in a CO2 laser. This consists of a tube filled with a mixture of CO2, nitrogen, and helium that is made to fluoresce by a high-voltage discharge. The tube emits infrared radiation with a wavelength of about 1.6 mm and is capable of delivering outputs up to 20 kW.
Welding and cutting processes used in shipbuilding
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Laser cutting relies on keyholing to penetrate the thickness, and the molten metal is blown out of the hole by a gas jet. A nozzle is fitted concentric with the output from a CO2 laser so that a gas jet can be directed at the work coaxial with the laser beam. The jet can be an inert gas, nitrogen, or in the case of steel, oxygen. With oxygen there is an exothermic reaction with the steel, giving additional heat as in oxy-fuel cutting. The thermal keyholing gives a narrow straight-sided cut compared with the normal cut obtained by other processes relying on a chemical reaction.
Water jet cutting The cutting tool employed in this process is a concentrated water jet, with or without abrasive, which is released from a nozzle at 2½ times the speed of sound and at a pressure level of several thousand bar. Water jet cutting can be used on a range of materials such as timber, plastics, rubber, etc., as well as steels and aluminum alloys. Mild steel from 0.25 to 150 mm in thickness and aluminum alloys from 0.5 to 250 mm in thickness can be cut. Being a cold cutting process the heat-affected zone, mechanical stresses, and distortion are left at the cut surface. Water jet cutting is slower than most thermal cutting processes and is not a portable machine tool.
Further reading ESAB advances in welding and cutting technology, The Naval Architect, July/August 2003. Exploiting friction-stir welding of aluminium, The Naval Architect, July/August 2004. Modern materials and processes for shipbuilding, The Naval Architect, July/August 2005. Smith BD: Welding Practice, 1996, Butterworth Heinemann.
Some useful websites http://www.rina.org.uk/tna.html Site for The Naval Architect magazine, with regular updates on ship and shipbuilding technology, including welding. www.esab.com Recommended for informative papers on welding processes and practices. www.controlwaterjet.co.uk Use, benefits, and capabilities of water jet cutting.